Photon counting systems employ sensors that react to photon inputs where inputs to the photon counting system are asynchronous. The number of photons arriving in unit time follows the Poisson distribution and the inter-arrival times between photons follows an exponential distribution. Theoretically, two photons may arrive with an infinitesimally small duration between them. Thus, it may not be possible to count all photons distinctly using a finite bandwidth system. To be able to count all photons distinctly, one would need an infinite bandwidth counting system, which is not practically realizable. The loss of counts due to a finite bandwidth counting system is not a problem as long as the system dead-time is well defined. The dead-time of a counting system may refer to the minimum separation in time between two incoming photons so that they are both recorded distinctly. For a non-paralyzable counting system, the input to output gain can be given by: n_m/n_T=1/(1+n_T*t_D), where n_m=Measured rate, n_T=True rate, and t_D=dead-time.
Counting systems with a higher dead-time are likely to cause more error owing to variation in dead-time and other non-idealities. Hence, to achieve suitable system performance, it is desirable that the dead-time be held as small as possible. The detector's response time also contributes to the overall dead-time. To control this value, the detector is generally biased with a very large reverse bias (e.g., up to 2000V). As a result, a current flows through the detector even when no x-ray photons are incident. This current is known as ‘dark current’. The current pulses that result when a photon is incident on the detector ride over this dark current. The dark current from the detector, if not compensated for, can cause dynamic range and energy resolution issues.